In recent years, there has been a demand for development of a technology of utilizing biomass of plants or plant-derived processed products, and a technology of converting biomass into petroleum-derived chemical products, resin products, and the like. For example, ligneous waste materials such as bark, lumber from thinning, and construction waste materials have been hitherto mostly disposed of. However, global environment protection has become an important problem to be solved, and from that point of view, studies have begun to be made on reutilization and recycling of ligneous waste materials.
Major components of general ligneous materials are cellulose, hemicellulose, and lignin. Among these, lignin is included at a proportion of about 30% in the ligneous materials, and since lignin has a structure richly containing aromatic rings, lignin can be utilized as an aromatic resin raw material.
Currently, investigations have been conducted on resin compositions, tires, and the like, in which lignin (lignin derivative) having a relatively low molecular weight that has been taken out (extracted) from biomass such as ligneous materials by various methods is utilized as a resin raw material (see, for example, PTL 1 and PTL 2).
Regarding the method for extracting a lignin derivative, the following method may be mentioned. For example, a delignification treatment performed at the time of pulp production or at the time of taking out a biofuel or a biomaterial from biomass; a solvent extraction treatment from residue and bagasse obtained after a saccharification treatment when a biofuel or a biomaterial is similarly taken out from biomass; a delignification treatment by means of a mechanical treatment; and a lignin extraction treatment by means of a high temperature high pressure water treatment, a steam blasting treatment, or an organosolv process may be used.
Lignin derivatives obtained as described above have highly polar structures richly containing phenolic hydroxyl groups or alcoholic hydroxyl groups. Therefore, investigations have been conducted on compositions and tires that utilize lignin derivatives as tackifiers and antioxidants (see, for example, PTL 1).
Meanwhile, lignin derivatives are also expected to be utilized as rubber reinforcing materials. Examples of a rubber reinforcing effect of lignin derivatives include an increase in the elastic modulus of rubber, a decrease in the hysteresis loss, and an increase in mechanical strength. These characteristics lead to enhancement of rigidity, low exothermic tendency, and mechanical strength of rubber component parts.
Furthermore, for example, in PTL 2, lignin is dissolved in black liquor containing sodium hydroxide and sodium sulfide, particulate lignin derivatives are collected from the black liquor containing the dissolved lignin, and the lignin derivatives are utilized as filler materials to be added to rubber compositions. However, in this case, such lignin derivatives do not have sufficient characteristics as filler materials, and cause a decrease in rigidity or mechanical strength of a rubber composition.
In addition to that, PTL 3 discloses that lignophenol, which is a product of addition of phenol to lignin, is produced by treating biomass using phenol and concentrated sulfuric acid, and lignophenol is utilized as a rubber reinforcing resin. However, although systems using various lignophenol derivatives are disclosed, these systems include large quantities of phenols, which are petroleum-derived components, and a rubber reinforcing effect for rigidity or the like is also insufficient.